cv
1.6 MHz
S
R
R
Q
+
-
+
-
+
-
-
+
EN
V
IN
Thermal
SHDN
SW
I
LIMIT
NMOS
UVLO = 2.3V
I
SENSE-AMP
Internal
Compensation
Soft-Start
Corrective - Ramp
Oscillator
Control Logic
V
REF
= 1.255V
FB
GND
11
LM2735-Q1
www.ti.com.cn
ZHCSIR2 –SEPTEMBER 2018
Copyright © 2018, Texas Instruments Incorporated
7.2 Functional Block Diagram
7.3 Feature Description
7.3.1 Current Limit
The LM2735-Q1 uses cycle-by-cycle current limiting to protect the internal NMOS switch. It is important to note
that this current limit will not protect the output from excessive current during an output short circuit. The input
supply is connected to the output by the series connection of an inductor and a diode. If a short circuit is placed
on the output, excessive current can damage both the inductor and diode.
7.3.2 Thermal Shutdown
Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature
exceeds 160°C. After thermal shutdown occurs, the output switch does not turn on until the junction temperature
drops to approximately 150°C.
7.3.3 Soft Start
This function forces V
OUT
to increase at a controlled rate during start-up. During soft start, the reference voltage
of the error amplifier ramps to its nominal value of 1.255 V in approximately 4 ms. This forces the regulator
output to ramp up in a more linear and controlled fashion, which helps reduce inrush current.
7.3.4 Compensation
The LM2735-Q1 uses constant-frequency peak current mode control. This mode of control allows for a simple
external compensation scheme that can be optimized for each application. A complicated mathematical analysis
can be completed to fully explain the internal and external compensation of the LM2735-Q1, but for simplicity, a
graphical approach with simple equations will be used. Below is a Gain and Phase plot of a LM2735-Q1 that
produces a 12-V output from a 5-V input voltage. The Bode plot shows the total loop Gain and Phase without
external compensation.
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